1. Field
Embodiments disclosed herein relate generally to shale shakers and screens for shale shakers. Specifically, embodiments disclosed herein relate to a shale shaker having pulse-vacuum assisted screening. Additionally, embodiments disclosed herein relate to methods and apparatus for removing entrained gases from a slurry.
2. Background Art
Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. The mud is mixed at the surface and pumped downhole through a bore of the drillstring to the drill bit where it exits through various nozzles and ports, lubricating and cooling the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) and the density (or its inverse, specific gravity) of the fluid used. Various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture for the type and construction of the formation to be drilled. Increasing the amount of weighting agent solute dissolved in the mud base will generally create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit stir up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud, and the cutting particulates must be removed before the mud can be recycled.
One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker.” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at the top of the shale shaker. As the drilling mud travels down the incline toward the lower end, the fluid falls through the perforations to a reservoir below thereby leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall off the lower end of the shaker table. The above described apparatus is illustrative of one type of shale shaker known to those of ordinary skill in the art. In alternate shale shakers, the top edge of the shaker may be relatively closer to the ground than the lower end. In such shale shakers, the angle of inclination may require the movement of particulates in a generally upward direction. In still other shale shakers, the table may not be angled, thus the vibrating action of the shaker alone may enable particle/fluid separation. Regardless, table inclination and/or design variations of existing shale shakers should not be considered a limitation of the present disclosure.
Preferably, the amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it may either return to service in the borehole immediately, be stored for measurement and evaluation, or pass through an additional piece of equipment (e.g., a drying shaker, a centrifuge, or a smaller sized shale shaker) to remove smaller cuttings and/or particulate matter.
Screens used with shale shakers are typically emplaced in a generally horizontal fashion on a generally horizontal bed or support within a basket in the shaker. The screens themselves may be flat or nearly flat, corrugated, depressed, or contain raised surfaces. The basket in which the screens are mounted may be inclined towards a discharge end of the shale shaker. The shale shaker imparts a rapidly reciprocating motion to the basket and hence the screens. Material from which particles are to be separated is poured onto a back end of the vibrating screen, flowing toward the discharge end of the basket. Large particles that are unable to move through the screen remain on top of the screen and move toward the discharge end of the basket where they are collected. The smaller particles and fluid flow through the screen and collect in a bed, receptacle, sump, or pan beneath the screen.
In some shale shakers a fine screen cloth is used with the vibrating screen. The screen may have two or more overlaying layers of screen cloth or mesh. Layers of cloth or mesh may be bonded together and placed over a support, supports, or a perforated or apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated by vibratory equipment to create a flow of trapped solids on top surfaces of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
While there are numerous styles and sizes of filter screens, they generally follow similar design. Typically, filter screens include a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids therethrough, while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are generally flat or slightly curved in shape, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. In theory, the pyramid-shaped channels provide additional surface area for the fluid-solid separation process to take place, and act to guide solids along their length toward the end of the shale shaker from where they are disposed.
The separation of drilling fluid and other solids from drill cuttings using a screen shaker is often incomplete, resulting in wet drill cuttings. As described above, the drilling mud is introduced to the top of the screen and allowed to flow downward through the screen by gravity alone. Often, additional equipment, such as additional screen separators, hydrocyclones, dryers, drying shakers, centrifuges, hydrocyclone shakers, thermal desorption systems, and other equipment, are used to further dry the cuttings and recover drilling fluid. For example, cuttings from a shale shaker may fall onto a rotary vacuum dryer, where the cuttings travel on a circumferentially rotating screen. Air may be used to strip drilling fluid off the cuttings and into the screen, such as by pulling a vacuum from the interior of the rotating screen (for example, the ROTAVAC™ Rotary Vacuum Dryer fluid recovery and cuttings drying system, available from Halliburton).
It is desired to improve the rate and efficiency at which shakers remove liquid from cuttings or other solids. To enhance the gravity-driven separation as described above, it is known that increasing the head on the shaker can increase the throughput of fluids through the screen. Increasing the pressure differential through the screen will likewise increase the fluid capacity of the shaker.
One example of a shaker with increased pressure differential is disclosed by Hensley et al. in U.S. Patent Application Publication No. 20050183994A1. Hensley et al. disclose an integrated, transportable cutting treatment system, where a pressure differential is developed across the screens to increase the flow rate of drilling mud through the screens. Hensley et al. use an air pump to develop a vacuum beneath the screens to draw mud through the screens. However, applying a continuous vacuum beneath a screen to draw fluid through the screen may result in solids sticking to the screen, hindering the conveyance of solids off the end of the shaker as needed, thereby preventing fluids from being filtered through the screen.
There exists a continuing desire for shakers having increased fluid capacity, increased fluid flow-through rates across the screens, and/or improved fluid removal efficiencies. Accordingly, there exists a need for a shaker with increased pressure differential. Preferably, the means used to increase the pressure differential do not substantially hinder the flow of solids across the screen deck. Additionally, there exists a need for a shaker for removing entrained gases from the recovered drilling fluid.